In the current UMTS (universal mobile telecommunication systems) specifications or the current WCDMA (wideband code division multiplexing access) specifications Release 7, MIMO (multiple input multiple output) antenna systems are introduced to enable high data rates to be achieved, especially in the downlink. In principle, the peak data rate in a WCDMA wireless communications system incorporating high-speed packet access (HSPA) is doubled when using MIMO compared to previous releases. HSPA is a new protocol for data transmissions and is a combination of HSDPA (high-speed downlink packet access) in the downlink and HSUPA (high-speed uplink packet access) in the uplink, both of which are specified in 3GPP Release 5/6. HSPA can provide data rates up to 14 Mbps in the downlink and 5.8 Mbps in the uplink. If MIMO technology is incorporated in HSPA, data rates up to 28 Mbps in the downlink are provided. The faster data speeds made possible with HSPA evolution will thus enhance the user experience for services like Mobile Broadband and Mobile TV while reducing the production cost per gigabyte.
HSPA and MIMO can be implemented in user equipments (UE) such as mobile phones or wireless terminals, as well as in network nodes such as a radio base station or a NodeB. In order to support MIMO technology in HSPA, control channels in the uplink and in the downlink have been modified. For the downlink, the downlink high-speed shared control channel (DL HS-SCCH) has been extended to incorporate the necessary information for transmitting up to two transport blocks of data simultaneously. Similarly, for the uplink, the high-speed dedicated physical control channel (UL HS-DPCCH) has been extended to incorporate channel quality indicators (CQI) for up to two streams in addition to pre-coding information. As well known in the art, a CQI is a measurement of the communication quality of wireless channels. CQI can be a value (or values) representing a measure of channel quality for a given channel realization. Typically, a high value CQI is indicative of a channel with high quality and vice versa. A CQI for a channel can be computed by making use of performance metric, such as a signal-to-noise ratio (SNR) or signal-to-interference plus noise ratio (SINR).
Due to the extension of the MIMO control channels, a coverage area for a terminal in a MIMO transmission/reception mode is somewhat smaller than that for a terminal in a non-MIMO mode. This is because of the larger payload in the MIMO control channels. Alternatively, more output power is required/needed for control channels to retain the same coverage when a terminal is entering a transmission/reception MIMO mode.
In the US patent application publication US 2006/0046662, a method and a system are provided for mitigating the effect of differing radio link qualities on the communication system wherein HSDPA and MIMO technology are implemented. In this prior art, a MIMO capable UE having a plurality of antennas can monitor its radio environment and send information about its radio environment to a radio base station (or a NodeB). The radio base station then adapts itself and the UE, in terms of receiver resources (such as the number of receiver antennas to be used) to cope with the varying radio conditions. The UE can also recommend to the base station how it should adapt itself, or it can also recommend that the base station uses a particular subset of antennas for transmitting data.
A drawback with the solution described above is that if the radio base station is to make a decision on how many transmit antennas to use, it would need to know all of the channel coefficients (including those of the interferer(s)) as well as the capability of the mobile terminal to cancel interference. This will therefore require a significant signalling load in the radio spectrum. A solution to that problem would be to let the mobile terminal make all or some of the decisions in order to reduce the signalling load. However, this would still require a significant amount of computational resources in the mobile terminal and power consumption at a mobile terminal or at the UE is a very important factor especially for MIMO capable mobile terminals (UEs).